Released: 1996, Nintendo 64
Super Mario 64 had the titular mustachioed plumber step into the third dimension, resulting in a very revolutionary and influential title for the time. It was massively praised for its expansive worlds and hidden secrets, and it still has a following to this day.
The game went through a lot of different ideas during development, and it's no surprise that there's more lurking underneath the surface. Highlights include Hoot's Egg, unusually-placed objects, and even a handful of regional differences. A multiplayer mode was even considered for the game where Luigi would have been a second playable character, but the released game has neither - to the point that the only remnant of Luigi is his shadow... in the final, at least.